Accessing every child class of parent class in Jav

问题:

I have to implement a logic whereby given a child class, I need to access its parent class and all other child class of that parent class, if any. I did not find any API in Java Reflection which allows us to access all child classes of a parent class. Is there any way to do it?

For example:

class B extends class A
class C extends class A

Now using class B, I can find the superclass by calling getSuperClass(). But is there any way to find all the child classes once I have the parent class i.e. class B and class C??

回答1:

If this wasn't homework (where 3rd party librares are probably not allowed), I would have suggested Google Reflections' Reflections#getSubTypesOf().

Set<Class<? extends A>> subTypes = reflections.getSubTypesOf(A.class);

You can do this less or more yourself by scanning the classpath yourself, starting with ClassLoader#getResources() wherein you pass "" as name.

回答2:

You are correct: there is no direct API for this. I guess you could scan all loaded classes and see if they are a subclass of a given class.

One problem: you'll only able to find classes that are already loaded. None of these methods will find classes that haven't been loaded yet.

回答3:

You can use:

thisObj.getClass().getSuperclass()
thisObj.getClass().getInterfaces()
thisObj.getClass().getGenericInterfaces()
thisObj.getClass().getGenericSuperclass()

I recently coded something to scan the members of a class and if those were non-primitives scan those as well. However, I did not traverse upwards so I didn't use the above methods but I believe they should do what you want.

EDIT: I ran a simple test with the following:

public class CheckMe {
  public CheckMe() {
  }
}

public class CheckMeToo extends CheckMe {
  public CheckMeToo() {
  }
}
// In main
System.out.println( CheckMeToo.class.getSuperclass() );

// Output
class CheckMe

After that it's a matter of coding the traversal. If its parametrized then things may get a little complicated but still quite doable.

EDIT: Sorry didn't read carefully, let me look further into it.

EDIT: There doesn't seem to be any way to do it without scanning everything in your CLASSPATH and checking to make sure an object is an instance of some class.